1. Field of the Invention
The present invention relates to a fixing device used in an image forming apparatus, such as a copier, a printer, or a facsimile machine, and an image forming apparatus including the fixing device.
2. Description of the Related Art
In recent years, an image forming apparatus, such as a copier, a printer, or a facsimile machine, forms images through an image forming process, such as electrophotography recording, electrostatic recording, or magnetic recording, and forms unfixed toner images on recording materials, such as recording material sheets, printing paper, photosensitive paper, or dielectric-coated paper, by an image transfer method or a direct method. Well-known examples of the fixing device for fixing an unfixed toner image include a device that performs a fixing process by applying heat and pressure to a toner image formed on a recording sheet at a nip between an endless belt and a pressing roller.
As the fixing device as described above, a belt fixing device is known. For example, as illustrated in FIG. 12, a fixing belt 103 is extended between a fixing roller 102 and a heating roller 104 in which a built-in heater 105 is provided, and a pressing roller 101 is pressed against the fixing roller 102 via the fixing belt 103 to form a fixing nip. A tension applying unit 107 applies tension to the fixing belt 103, a separation claw 106 is provided to separate a recording sheet P from the fixing belt 103 after fixing, and a thermistor 110 detects a temperature of the fixing belt 103 on the heating roller 104 side.
Incidentally, in recent years, there is a need to further reduce a warm-up time (a time needed until a predetermined temperature (reload temperature) that enables printing is reached from a room temperature at the time of power activation), or a first print time (a time needed until printing operation is performed through printing preparation and ejection of a sheet is complete upon reception of a print request). Further, with an increase in the speed of the image forming apparatus, the number of sheets to be fed per unit time increases, in particular, a necessary amount of heat increases, so that the amount of heat may become insufficient especially at the beginning of continuous printing (so-called temperature drop). However, the conventional belt fixing device is unable to sufficiently cope with this issue.
Therefore, a fixing device using a pipe-shaped metallic heat conductor has been proposed, in which a pipe-shaped metallic heat conductor is arranged inside an endless belt and the inner side of the metallic heat conductor is heated by a heat source to heat the entire belt with convection, radiation heat, or heat conduction in air between the endless belt and the metallic heat conductor. The fixing device using the pipe-shaped metallic heat conductor is disclosed in, for example, Japanese Laid-open Patent Publication No. 2007-334205.
The fixing device as described above further includes a pressing roller that comes in contact with the metallic heat conductor via the endless belt to form a nip portion, and the endless belt is moved in a circumferential direction with respect to the fixed metallic heat conductor along with rotation of the pressing roller. This configuration makes it possible to heat the entire endless belt included in the fixing device, reduce the first print time from a heating standby state, and resolve a shortage of heat at the time of high-speed rotation.
Further, Japanese Laid-open Patent Publication No. 2007-233011 discloses an example of a fixing device that includes, on the inside of an endless belt, only a nip forming member having a sliding surface that comes in sliding contact with the endless belt, a backup member that is located on the side opposite to the sliding surface of the nip forming member and that supports the nip forming member, and a heat source.
In this example, a pressing roller is provided that forms a nip portion between itself and the sliding surface of the nip forming member to nip and press the endless belt and that causes the endless belt to rotate in a predetermined direction. In the fixing device, it is possible to reduce a heat capacity around the heat source. Further, the heat source directly heats the endless belt and the nip is formed at the heated region, so that it is possible to reduce a first print time from a heating standby state.
Incidentally, as described in Japanese Laid-open Patent Publication No. 2007-233011, in the fixing device that includes, on the inside of the endless belt, the nip forming member having the sliding surface that comes in sliding contact with an inner surface of the endless belt and having the backup member that supports the nip forming member, the backup member needs to penetrate through the belt. In this case, the nip supporting member serving as the backup member for receiving a load transmitted from the pressing roller via the nip forming member functions as a doubly-supported beam that supports the received load at both ends.
The nip supporting member serving as the backup member that functions as the doubly-supported beam needs to support a load received from the pressing roller and to form a nip width with high accuracy, so that the nip supporting member needs to have a predetermined volume to ensure the rigidity in order to maintain the strength as much as possible.
However, it is preferable to reduce the volume of the nip supporting member as much as possible in order to downsize the fixing device and reduce an extra heat capacity. In this regard, however, if the strength of the nip supporting member is reduced with reduction of the volume, the center of the nip supporting member including the sliding surface may be deflected in a direction away from the nip surface, so that the uniform nip width in the longitudinal direction may not be obtained.
Therefore, as a method of reducing the volume of the nip supporting member while maintaining the strength of the nip supporting member as much as possible, a method of forming the nip supporting member in a box shape may be applicable, for example. However, if a shape formed by simply bending a plate into a box shape is employed, it is difficult to ensure the dimensional accuracy of the nip supporting member, so that it is difficult to accurately determine a fixing nip position.
As descried above, the endless belt of the fixing device includes, on the inside thereof, the nip supporting member serving as a doubly-supported beam that supports a load applied from a pressing member, such as the pressing roller, via the nip forming member. In this case, there is a need to make a structure, in which the nip supporting member has an adequate strength, in which the volume of the nip supporting member is reduced, and in which the uniform nip width in the longitudinal direction as the dimensional accuracy of the supported nip forming member can easily be ensured.
Namely, in view of the above-described conventional arts, there is a need to provide a fixing device and an image forming apparatus configured such that the dimensional accuracy of a nip supporting member, which supports a nip forming member forming a fixing nip with an opposing pressing member and which determines a nip position, can easily be ensured to stabilize a nip width.